This invention is concerned with the purification of bisphenol-A (or BPA), the most common species being 2,2-bis(4-hydroxyphenyl)propane. More particularly, this invention is directed to a method of recovering bisphenol-A in a purified state from crude bisphenol-A by crystallization in phenolic water of a particular phenol/water ratio.
Crude bisphenol-A is the isolated product of commercial processes for preparing bisphenol-A. It is a mixture of bisphenol-A and impurities derived from a BPA synthesis reaction. An example of a BPA synthesis reaction is the acid-catalyzed condensation of phenol and acetone, where phenol and acetone react in the presence of an acidic material, such as sulfuric acid, hydrochloric acid, cation exchange resin, etc.
The crude bisphenol-A produced contains undesirable impurities such as phenol, 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl)propane having the formula ##STR1## a trishydroxyphenyl compound of the formula ##STR2## and small amounts of other impurities such as the two compounds having formulas ##STR3## and some impurities which discolor the crude bisphenol-A with unknown structure (herein identified as color bodies).
Since bisphenol-A is used in making polycarbonate resins by reaction of the former with either phosgene or diphenyl carbonate, or for making epoxy resins, both resins being used extensively in commercial applications involving molding, casting and sheet forming processes, it is highly important that the monomeric bisphenol-A used to make such resins be as pure as possible in order to avoid adverse effects on the properties of the polymers thus obtained.
The preparation of bisphenol-A by the acid catalyzed reaction of phenol and acetone is usually carried out in excess phenol (2 or more moles per mole of acetone). This reaction mixture is typically subjected to a series of distillation steps to remove the excess phenol, acetone and water or the acetone and water are removed and the bisphenol-A/phenol adduct is crystallized from excess phenol, followed by stripping of the phenol. Both procedures provide liquid crude bisphenol-A which is the starting point for making bisphenol-A of high purity. Distillation or stripping of the phenol from the reaction mixture can be carried out only to a limited extent on account of the thermal instability of the bisphenol-A product.
A conventional method of recovering pure bisphenol-A product from crude bisphenol-A involves crystallization in the presence of an organic solvent. The crude molten bisphenol-A is first dissolved in a suitable organic solvent and the solution is then cooled to yield pure bisphenol-A crystals, which are recovered by filtration. However, this process suffers in that (1) the crystals produced are fine, powdery and needle-like and are difficult to handle, store and dry; (2) these crystals permit some organic solvent to occlude during crystallization, which cannot be removed during the drying step and hence is present in the polymerization process and (3) these processes require the use of a foreign organic solvent. The expression "foreign organic solvent" being defined herein as any organic solvent which is not utilized in the synthesis of bisphenol-A. These are typically organic solvents other than phenol and acetone. The use of foreign organic solvents is undesirable since they must be dealt with separately when isolating pure bisphenol-A.
A method which uses water as a crystallization medium for crude bisphenol-A is described in U.S. Pat. No. 3,326,986. According to this patent, the isolated crude bisphenol-A in molten form is purified by first mixing with water and cooling the mixture to yield large, rhombic crystals of bisphenol-A. The crystallization in water does not provide purification; however, separation of these crystals from the mother liquor, followed by an organic solvent wash, results in purified bisphenol-A. Although the process described within the above-referenced patent avoids occlusion by an organic solvent during the crystal formation step and yields large, less needle-like crystals that are easy to handle, quantities of a foreign organic solvent are utilized to wash the crystals.
It is desirable to obtain a high degree of purification without the use of a foreign organic solvent and maintain the large, rhombic character of the aqueous crystallized bisphenol-A.
The process described in copending allowed application Ser. No. 443,344, describes a washing procedure for purifying aqueous crystallized bisphenol-A with a water/organic solvent wash, the organic solvent being water immiscible. Although the process provides large, purified crystals of bisphenol-A which are easy to handle, the use of a "foreign organic solvent" is required. The process comprising this invention obtains large, highly purified bisphenol-A crystals by utilizing phenolic water (phenol being a non-foreign organic solvent) of a particular phenol/water ratio as a crystallization solvent. Although Luten describes the "breaking" of a bisphenol-A phenol adduct with water in U.S. Pat. No. 2,791,616, large amounts of phenol and water are used with little or no purification, as shown in Example VIII below and the BPA crystals obtained are fine and needle-like. Reinitz describes the use of a phenol/water mixture with a foreign organic solvent as a crystallization medium for bisphenol-A in U.S. Pat. No. 4,192,955. This process does not provide bisphenol-A having a crystal morphology which matches that provided by an aqueous crystallization. Comparing Examples VIII and IX to the Examples which illustrate the process comprising this invention demonstrates the substantial improvement in bisphenol-A crystal morphology and purity over the purification processes described in these references which utilize phenol and water both with and without a foreign organic solvent. In addition, the process comprising this invention obtains high yields of purified bisphenol utilizing small quantities of water and phenol and no foreign organic solvents.